The human heart is divided into four chambers. These include the right atrium, the right ventricle, the left atrium, and the left ventricle. The right atrium and right ventricle are divided from the left atrium and left ventricle by a muscular wall called the septum. The atrial septum is the wall separating the atria and the ventricular septum is the wall separating the ventricles.
On the right atrial side of the atrial septum is a thin walled recessed portion of septal tissue called the fossa ovalis. In the heart of a fetus, the fossa ovalis is open and is called a foramen ovale. The foramen ovale is a small hole located in the atrial septum that is used during fetal circulation to speed up the travel of blood through the heart. Thus, blood can travel from the veins to the right side of the fetal heart and cross to the left side through the foramen ovale, bypassing the fetus's lungs.
Normally, the foramen ovale closes at birth when increased blood pressure on the left side of the heart forces the opening close. If the atrial septum does not close properly the resulting condition is called a patent foramen ovale (PFO). The PFO condition works like a valve, opening when increased pressure in the chest occurs. In some instances, this increased pressure can be caused by a valsalva maneuver. The valsalva maneuver can occur when people strain while having a bowel movement, a cough, or a sneeze.
During a valsalva maneuver, blood pressures within the right atrium can increase to a point at which blood may travel from the right atrium to the left atrium. If there is a clot or particles in the blood traveling in the right side of the heart, it can cross the PFO and enter the left atrium. The clot or particles can then travel out of the heart and to the brain (causing a stroke) or into a coronary artery (causing a heart attack).